Premium Member

.apk(Application File Package) files are essentially zip files with all the necessary files and data to install an application onto your Android device. They use the JAR (Java Archive) file format but they are built on the .zip format with the .apk suffix. Basically all Android applications are distributed in the .apk format for simplicity and portability.

Just an FYI, OP, what you're wanting to do is piracy. Developers work to create apps and generally charge very little for them, so I would suggest you go through Android MArket for any of your paid app needs. Piracy really kills the ecosystem and is theft from the devs, so I don't think anyone should help you out with this endeavor. No offense intended.

Premium Member

.apk is the file extension for Android applications and programs, think .exe for windows or .dmg for Mac OS X. All install-able apps on Android will have the extension of .apk.

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Again, to clarify...as I said before it's most frequently a compressed (zipped) package of files and folders. Yes, there are APK files that are self-executing and require NO installation, but the file format was designed as an installation package more so rather than an executable. Just like there are EXE files that can run stand-alone and then there EXE installers, there are APKs that can run stand-alone but the greater portion are installation packages that contain several components (see below).

Android application package file (APK) is the file format used to distribute and install application software and middleware onto the Google's Android operating system. To make an APK file, a program for Android is first compiled, and then all of its parts are packaged into one file. This holds all of that program's code such as (.dex files), resources, assets, certificates, and manifest file. As is the case with many file formats, APK files can have any name needed, but must end with the four character, three letter extension, .apk.[SUP][1][/SUP][SUP][2][/SUP][SUP][3][/SUP][SUP][4]
[/SUP]APK files are ZIP file formatted packages based on the JAR file format, with .apk file extensions. The MIME type associated with APK files is application/vnd.android.package-archive.[SUP][5]

[/SUP]An APK file is an archive that usually contains the following folders:​

AndroidManifest.xml: An additional Android manifest file, describing the name, version, access rights, referenced library files for the application. This file may be in Android binary XML that can be converted into human-readable plaintext XML with tools such as AXMLPrinter2 or apktool.​